JP2001164332A - Cr-Mo STEEL HAVING HIGH TOUGHNESS AND REHEAT CRACKING RESISTANCE AND PRODUCING METHOD THEREFOR - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce 1 to 1 1/4 Cr-Mo steel capable of obtaining exellent strength, toughness and weld cracking resistance even under PWHT conditions high in a tempering parameter. SOLUTION: Steel having a composition containing, by mass, 0.03 to 0.15% C, 0.01 to 0.8% Si, <=1% Mn,<=0.015% P, <=0.03% S, 0.8 to 1.6% Cr, 0.2 to 1% Mo, 0.002 to 0.005% Ti, 0.0003 to 0.003% B, 0.01 to 0.04% Al, <=0.006% N and <=0.003% O, moreover containing, as selective components, Cu, Ni, Nb, Ca, Mg and rare earth metals, and the balance Fe with inevitable impurities is held at 700 deg.C to the Acl transformation temp for <=30 min before normalizing and is thereafter subjected to tempering.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Cr-Mo steel used for chemical plant equipment such as petrochemical and petroleum refining and power plant equipment such as a boiler and nuclear power, and particularly has excellent toughness under PWHT conditions having a large tempering parameter. Cr-Mo steel and a method for producing the same.

[0002]

2. Description of the Related Art At present, pressure vessels, boiler piping, etc.
1Cr-1 / 2Mo steel typified by IS SCMV2 and 1 / 4Cr-1 / 2Mo steel typified by SCMV3 are widely used. However, these steel grades are JI
Compared to 21 / 4Cr-1Mo steel typified by SSCCMV4, the weldability and cost are superior, but the toughness is inferior. Especially when thick materials and accelerated cooling cannot be performed after normalizing,
Alternatively, when receiving PWHT for a long time at high temperature, the deterioration is remarkable, and improvement is an issue. In addition, these steel types have a relatively high reheat cracking susceptibility or creep embrittlement susceptibility of a welded portion among Cr-Mo steels, which may cause a problem.

[0003] 1Cr-1 / 2Mo steel, 1 1 / 4Cr-
As a technology to improve the toughness of 1 / 2Mo steel,
Japanese Patent Publication No. 50547 discloses a technique for reducing the amount of C and adding B. However, although the reduction of the C content is effective for improving the toughness, the strength and toughness of a thick material in the case of a high temperature and a long time in which the PWHT condition exceeds 20.3 × 10 ³ in the tempering parameter of the equation (1) is used. Is more difficult to secure.

[0004] T. P. = (T + 273.15) (logt + 20) (1) However, T: temperature (° C.), t: time (h).
Ti-B-low-N system with Ti added and low N to ensure hardenability due to 1Cr-1 / 2M
In the microstructure obtained by normalizing the o steel and the 1 / 4Cr-1 / 2Mo steel, high toughness is not so stably obtained.

[0005] Further, A for fixing N with Al instead of Ti
Although 1-B-low N system has been proposed, Al cannot fix N completely at a normalizing temperature near 900 ° C. and is unstable, so that it is difficult to secure stable hardenability and toughness is also high. May not be constant. Further, both Al and Ti are elements that increase the reheat cracking susceptibility of the welded portion, and it is not always desirable to add them.

[0006]

As described above, in the prior art, 1Cr-1 / 2Mo steel, 1 1 / 4Cr-1 /
2Mo steel is a thick material and high temperature and long time PWHT
However, it is difficult to stably combine the excellent toughness and the excellent reheat cracking resistance under the conditions required.

Therefore, the present inventors have developed a PWHT condition (for example, 690 ° C. × 24 h) having a particularly large tempering parameter.
r, T. P. = 20.6 × 10 ³ ), excellent strength and toughness can be obtained, and 1-1 having excellent reheat cracking resistance.
It is an object to provide a 1 / 4Cr-1 / 2Mo steel and a method for producing the same.

[0008]

Means for Solving the Problems The present inventors have proposed a Cr-M
Intensive studies were conducted on the effects of the composition of the o-based steel and the manufacturing conditions on the toughness and reheat crack resistance, and the following findings were obtained.

1. The addition of Ti and B and the reduction of N are effective for hardenability. However, since Ti has an adverse effect on toughness, it is necessary to reduce the amount of Ti added.

[0010] 2. In order to ensure hardenability in low Ti system, addition of Al is effective. It is low Ti, Al-B system mixed with appropriate amount of Al, and other components are optimized, toughness, reheat crack resistance. It is possible to obtain an excellent component composition.

3. The effect of the above component composition is 70
It is more desirable to carry out the heat history held in the temperature range of 0 ° C. to the Ac1 transformation point for 30 minutes or more as an intermediate tempering before normalizing, or to carry out the heat history at the time of heating for normalizing. .

The present invention has been made by further studying these findings. That is, the present invention provides: In mass%, C: 0.03 to 0.15%, Si:
0.01 to 0.8%, Mn: 1% or less, P: 0.015
%, S: 0.003% or less, Cr: 0.8 to 1.6
%, Mo: 0.2-1%, Ti: 0.002-0.00
5%, B: 0.0003-0.003%, Al: 0.0
1 to 0.04%, N: 0.006% or less, O: 0.00
Cr-Mo steel containing not more than 3% and excellent in toughness and reheat cracking resistance composed of balance Fe and inevitable impurities.

2. Further, in mass%, Cu: 0.05 to
A Cr-Mo steel containing 0.5% and having excellent toughness and reheat crack resistance according to 1 above.

3. Further, in mass%, Ni: 0.05 to
3. The Cr-Mo steel according to 1 or 2, which contains 0.8% and has excellent toughness and reheat crack resistance.

4. Further, in mass%, Nb: 0.005
4. The Cr-Mo steel according to any one of 1 to 3, which contains -0.03% and has excellent toughness and reheat crack resistance.

5. Furthermore, in mass%, Ca: 0.000
5 to 0.005%, Mg: 0.0005 to 0.005
%, REM: Cr-Mo steel excellent in toughness and reheat crack resistance according to any one of 1 to 4 containing one or more of 0.0005 to 0.02%.

The steel having the composition described in any one of 6.1 to 5 is maintained in a temperature range of 700 ° C. to the Ac1 transformation point for 30 minutes or more before normalizing, and is tempered after normalizing. A method for producing a Cr-Mo steel having excellent toughness and reheat cracking resistance.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION The chemical components of the present invention will be described.

C: 0.03 to 0.15% C is added in an amount of 0.03% or more in order to secure strength and toughness even in a thick material having a thickness of 100 mm or more. 0.15
%, The toughness and the low-temperature weld cracking property are deteriorated, so 0.03 to 0.15% is added.

Si: 0.01 to 0.8% Si is effective for securing strength and improving oxidation resistance.
% Or more. When added in excess of 0.8%, the toughness is degraded, and the tempering sensitivity is increased, so that the content of 0.01% to
0.8%.

Mn: 1% or less Mn is effective in improving the strength and toughness, but at the same time, it is made 1% or less in order to increase the temper embrittlement susceptibility.

P: 0.015% or less, S: 0.003%
The content of P is set to 0.015% or less in order to promote temper brittleness, impair toughness, and increase reheat cracking sensitivity. S is also set to 0.003% or less in order to increase the reheat cracking sensitivity.

Cr: 0.8 to 1.6% Cr is effective for high-temperature strength, hydrogen erosion resistance and oxidation resistance, which are important as high-temperature steel, and it is necessary to add 0.8% or more. If it exceeds 1.6%, the cost increases and the weldability deteriorates, so the content is set to 0.8 to 1.6%.

Mo: 0.2-1% Mo stably forms carbides, and is effective for high-temperature strength, creep strength and hydrogen erosion resistance, like Cr.
Add 2% or more. If added in excess of 1%, the weldability is impaired and the economy is impaired, so the content is made 0.2-1%.

Ti: 0.002 to 0.005% Ti is a strong nitride forming element, and is added in order to improve the hardenability of B by fixing N. Cr
In the case of a Cr-Mo steel having an amount of about 1 to 1.6%, the addition of a large amount lowers the toughness.
Add up to 05%.

B: 0.0003% to 0.003% B is added in an amount of 0.0003% or more to improve hardenability. If added in excess of 0.003%, the weldability, toughness, and reheat cracking resistance of the welded portion are reduced.
3 to 0.003%.

Sol. Al: 0.04% or less sol. Al is necessary as a deoxidizing material and is effective for fixing N, but is set to 0.04% or less to reduce the reheat cracking susceptibility and high-temperature ductility of the welded portion.

N: 0.006% or less N binds to B, reduces the amount of solid solution B, and impairs the effect of improving the hardenability of B, so is limited to 0.006% or less.

O: not more than 0.005% O is not more than 0.005% in order to form oxide-based inclusions to reduce ductility and weldability, and to make the effect effective when Ca is added. Restrict to

The above is the basic composition of the steel of the present invention. In the case of further improving the properties, Cu, N
One, two or more of i, Nb, Ca, Mg, and REM can be added.

Cu: 0.05 to 0.5% Cu is added in an amount of 0.05% or more in order to enhance the hardenability and to increase the strength as a solid solution strengthening element. When added in excess of 0.5%, the reheat cracking sensitivity is increased, and the creep ductility and hot workability are reduced.

Ni: 0.05 to 1% Ni is 0.05% to improve hardenability and improve toughness.
Add above. If the addition exceeds 1%, the cost increases and the susceptibility to reheat cracking increases, so the content is set to 0.05 to 1%.

Nb: 0.005 to 0.03% Nb is a stable carbonitride forming element, and is added in an amount of 0.005% or more to improve room temperature, high temperature strength and creep rupture strength. If added in excess of 0.03%, toughness and weldability are reduced, and reheat cracking sensitivity is increased.
0.03% or less.

Ca: 0.0005-0.005%, M
g: 0.0005-0.005%, REM: 0.000
One or more of 5 to 0.02% In order to further reduce the reheat cracking sensitivity, one or more of Ca, Mg and REM are added. Ca and Mg are strong sulfide-forming elements, fix solid solution S in steel, further control the distribution of oxide-based inclusions, and reduce the reheat cracking susceptibility. 0.005
% Or less.

REM whose atomic number is from 57 to 71
(Rare earth element) also reduces the susceptibility to reheat cracking by the same effect as Ca and Mg. In the case of adding, one or more of REMs (rare earth elements) having an atomic number of 57 to 71 are 0.0005% or more and 0.02% or less.

Next, the manufacturing conditions of the steel of the present invention will be described. The steel of the present invention is manufactured by normalizing and tempering. Normalization is performed after hot rolling or hot forming, liquid cooling or accelerated cooling depending on the required mechanical properties, and then tempering.

In the case of the steel of the present invention, in particular, in order to secure quenchability stably and to secure high strength without lowering toughness, the step of holding the steel in a temperature range from 700 ° C. to the Ac1 transformation point for 30 minutes or more is performed. Perform before the break-in. In the present invention, “before normalizing” means to perform before or during the temperature rise of normalizing.

In order to secure the hardenability stably, AlN is efficiently precipitated in addition to TiN precipitated as rolled to fix N. The holding temperature is lower than 700 ° C. because the diffusion rate of Al is low and the precipitation is insufficient on the low temperature side.
° C or higher, and below the Ac1 transformation point to facilitate the precipitation of AlN. The holding time is set to 30 minutes or more so that AlN is sufficiently precipitated.

[0039]

Embodiments of the present invention will be described below.

The laboratory ingots of various components shown in Table 1 were rolled to a plate thickness of 20 mm to produce steel plates. Rolling is 1200
C., and the rolling was performed at a finishing temperature of about 1000.degree. C., and thereafter, normalizing at 920.degree. C. and tempering at 710.degree. C. were performed under various conditions. In addition, the cooling rate at the time of normalization was adjusted so as to be the air cooling rate at 1 / 2t of the plate thickness of 120 mm.

The post-weld heat treatment (PWHT) was performed at a high temperature of 690 ° C. × 24 hours, which is a severe condition for strength and toughness. The T.D. P. (Temper parameter) is equivalent to 20.6 × 10 ³ .

T. P. = (T + 273.15) (logt + 20) The tensile test was performed at room temperature and 450 ° C. at a high temperature of 6φ × 3.
The test was carried out using a 0GL test piece, the toughness was determined by a full curve by a Charpy impact test, and the fracture surface transition temperature (vTs
° C). All tests were performed in the 1/2 t, C direction.

The reheat cracking susceptibility test was conducted according to JIS Z 31
After the test bead welding, a post-weld heat treatment at 650 ° C. × 8 h was performed in accordance with the y-type weld cracking test method of No. 58. Thereafter, the average crack rate of the cross-sectional crack rates of the five sections was determined and evaluated.

The steel No. of the test steels shown in Table 1 Steel Nos. 1 to 10 are steels of the present invention. 1 to 8 are 1 1 / 4Cr-1 / 2M
o steel as a basic component. 2 is Ni-added system, steel N
o. No. 3 is a Cu, Ni added system, steel No. 3 5 is an Nb addition system,
Steel No. 6, 7 and 8 are Ni-Ca type and Ni-M, respectively.
g-based and Ni-REM-based.

Steel No. 9, steel No. 10 is 1Cr-1 /
2Mo steel as a basic component. Reference numeral 10 denotes a Cu-Ni added system. Steel No. Any of elements 11 to 20 is a comparative steel outside the scope of the present invention. In addition, steel No. 1-2
The Ac1 transformation point of 0 is 760 to 800 ° C.

Table 2 shows heat treatment conditions and material properties. The heat treatment conditions A are normalizing and tempering, but the heating rate during normalizing is slow, and the temperature is changed from 700 ° C. to the Ac1 transformation point during heating.
Heat for more than a minute. In the heat treatment condition B, the heating rate during normalizing is high, and the heating is not performed from 700 ° C. to the Ac1 transformation point during heating for 30 minutes or more. However, it is kept at 750 ° C. for 30 minutes before heating for normalizing.

The heat treatment condition C is set at 7 during heating during normalizing.
The temperature is maintained at 50 ° C. for 30 minutes, and the heat treatment conditions A, B and C are all the method according to the sixth aspect. On the other hand, the heat treatment conditions D and E are out of the range of the present invention. In heat treatment conditions A, B and C, steel No. All of the invention steels 1 to 10 have high normal-temperature strength, high-temperature strength, excellent toughness and good reheat crack resistance. Steel No. In the case of heat treatment conditions D and E, the sample was not heated to 700 ° C. to the Ac1 transformation point for more than 30 minutes, and the Charpy impact characteristics were inferior to those of heat treatment conditions A, B and C, but the reheat cracking resistance was poor. It is good, and has properties more than conventional steel.

Steel No. 11 to 20 are components outside the scope of the present invention, and the material properties are inferior even depending on the heat treatment condition A. Steel No. Sample No. 11 has low Ti and does not have sufficient hardenability, low room temperature strength and high temperature strength, and is inferior in toughness. Steel No. No. 12 has a high Ti addition amount, has sufficient hardenability, and has high strength at room temperature and high temperature, but is inferior in toughness. Steel No. No. 13 has higher Ti, lower toughness, and lower reheat cracking resistance.

Steel No. No. 14 has a high N content, and steel No. 14 has a high N content. Fifteen
Is not sufficient in quenchability due to low Al content, low in room temperature strength and high temperature strength, and inferior in toughness. Steel No. 1
All of Nos. 6 and 17 have a high Al content, high strength and high-temperature strength and ordinary toughness, but have deteriorated reheat cracking resistance. Steel No. No. 18 has a high oxygen content, and steel No. 18 has a high oxygen content. No. 19 has V added, and steel No. Sample No. 20 has a high S content, deteriorates toughness, and has extremely poor reheat cracking resistance.

[0050]

[Table 1]

[0051]

[Table 2]

[0052]

[Table 3]

[0053]

According to the steel of the present invention, the effect of improving the hardenability by B is stabilized and effective by adjusting the components and the production conditions, and the PWHT condition having a large tempering parameter is obtained by the component having excellent reheat cracking resistance. Also, excellent strength / toughness and weld crack resistance can be obtained.

Claims (6)

[Claims] C: 0.03 to 0.15% by mass%,
Si: 0.01 to 0.8%, Mn: 1% or less, P: 0.
015% or less, S: 0.003% or less, Cr: 0.8 to
1.6%, Mo: 0.2-1%, Ti: 0.002-
0.005%, B: 0.0003-0.003%, A
l: 0.01 to 0.04%, N: 0.006% or less,
O: Cr-Mo containing 0.003% or less, and excellent in toughness and reheat cracking resistance consisting of the balance of Fe and inevitable impurities.
steel. 2. Further, in mass%, Cu: 0.05 to 0.1.
The Cr-Mo steel according to claim 1, which contains 5% of the toughness and reheat crack resistance. 3. Ni: 0.05 to 0.1% by mass.
The Cr-Mo steel according to claim 1 or 2, which has an excellent toughness and reheat cracking resistance. 4. Nb: 0.005 to 5% by mass.
The Cr-Mo steel according to any one of claims 1 to 3, which contains 0.03% and has excellent toughness and reheat crack resistance. 5. Further, in mass%, Ca: 0.0005 to
0.005%, Mg: 0.0005 to 0.005%, R
The Cr-Mo steel excellent in toughness and reheat crack resistance according to any one of claims 1 to 4, which contains one or more of EM: 0.0005 to 0.02%. 6. A steel having a composition according to claim 1, wherein the steel has a temperature range of 700 ° C. to the Ac1 transformation point before normalizing.
A method for producing a Cr-Mo steel having excellent toughness and reheat crack resistance, characterized by holding for 0 minutes or more, normalizing, and then tempering.